1. Field of the Invention
The present invention relates to a membrane electrode assembly (MEA) and its manufacturing method, and further, a fuel cell which includes the MEA. More specifically, the present invention relates to an MEA which has a high level of power generation performance under a condition of low humidification, and its manufacturing method and a proton exchange membrane fuel cell (PEMFC) using the same.
2. Description of the Related Art
Fuel cells are power generation systems which produce electric power along with heat. A fuel gas including hydrogen and an oxidant gas including oxygen reacts together at electrodes containing a catalyst so that the reverse reaction of water electrolysis takes place in a fuel cell. Fuel cells are attracting attention as a clean energy source of the future since they have advantages such as a small impact on the environment and a low level of noise production relative to conventional power generation systems. Fuel cells are divided into several types according to the employed ion conductor. A fuel cell which uses a proton-conductive polymer membrane is called a proton exchange membrane fuel cell (PEMFC).
Among various fuel cells, a PEMFC, which can be used at around room temperature, is considered as a promising fuel cell for use in a vehicle and a household stationary power supply etc. and is being developed widely in recent years. A joint unit which has a pair of electrode catalyst layers on both sides of a polymer electrolyte membrane and which is called a membrane electrode assembly (MEA) is arranged between a pair of separators, on which either a gas flow path for supplying a fuel gas including hydrogen to one of the electrodes or a gas flow path for supplying an oxidant gas including oxygen to the other electrode is formed, in the PEMFC. The electrode for supplying a fuel gas is called a fuel electrode or cathode (electrode), whereas the electrode for supplying an oxidant gas is called an air electrode or anode (electrode). Each of the electrodes includes an electrode catalyst layer, which has stacked polymer electrolytes with carbon particles on which a catalyst such as a noble metal of platinum group is loaded, and a gas diffusion layer which has gas permeability and electron conductivity.
In order to improve output density of the fuel cell a number of attempts have been made to increase gas diffusion properties with respect to the electrode catalyst layers. A plurality of materials passes through pores in the electrode catalyst layer which are associated with a separator via a gas diffusion layer. The electrode catalyst layer on the fuel electrode supplies not only the fuel gas to the three-phase interface, at which the electrode reaction occurs, but also supplies water, which serves to smoothly conduct a produced proton in the polymer electrolyte membrane. The electrode catalyst layer on the air electrode removes water produced by the electrode reaction as well as supplies the oxidant gas.
In order to prevent a so-called flooding phenomenon, in which the power generation reaction stops by a hindrance of material transport, a number of techniques to improve drainage properties have been developed with respect to the MEA (See, for example, patent document 1-4 below).
In addition, among various problems to be solved to commercialize a PEMFC such as improvement of output density and durability, the most essential problem is cost reduction.
Reducing a humidifier is one way to reduce cost. Perfluorosulfonate membranes and hydrocarbon membranes are widely used as the polymer electrolyte membrane located in the center of an MEA. Since it is supposed to be necessary to adjust the moisture condition of such a polymer electrolyte membrane to almost a saturated water vapor pressure atmosphere in order to obtain excellent proton conductivity, water vapor is supplied by a humidifier from the exterior in the present technology. Thus, for the purpose of low power consumption and simplification, polymer electrolyte membranes which require no humidifier and have sufficient proton conductivity even under a low humidification condition are under development.
However, since an electrode catalyst layer with improved drainage properties makes a polymer electrolyte membrane dry under a low humidification condition, it becomes necessary to optimize a structure of the electrode catalyst layer to improve water retention properties. For example, a method of interposing a humid control film between the electrode catalyst layer and the gas diffusion layer has been proposed to improve water retention properties of a fuel cell under a low humidification condition.
For example, patent document 5 cited below discloses a method in which a humid control film made of conductive carbon powder and polytetrafluoroethylene prevents the polymer electrolyte membrane from drying by the humid control function. In addition, patent document 6 discloses a method of forming chases on the electrode catalyst layer's surface which contacts the polymer electrolyte membrane so that a decrease in power generation performance under a low humidification condition is suppressed by forming chases of 0.1-0.3 mm in width.    <Patent document 1>JP-A-2006-120506    <Patent document 2>JP-A-2006-332041    <Patent document 3>JP-A-2007-087651    <Patent document 4>JP-A-2007-080726    <Patent document 5>JP-A-2006-252948    <Patent document 6>JP-A-2007-141588
There is a problem, however, in these MEAs described in the patent document 1-6 that they do not have a satisfactory power generation performance. In addition, manufacturing methods of these also have a problem of complexity and high-cost.